Monitoring carried out by Strathclyde Partnership for Transport has found only 28% of homes within its area are now within walking distance of a bus stop with a frequent service, down from 44% in 2009.
The findings are thought to mirror service cuts across Scotland.
The drop has prompted fresh concern about isolation in communities without public transport links, and prompted calls for a radical shake-up in the way buses serve towns and cities in the current economic climate.
The report shows the crisis in the bus industry since 2008 has accelerated over the last year thanks to soaring fuel costs and cuts in government subsidies.
Despite a resurgence in rail travel, buses are still by far the most-used public transport. About 440 million bus journeys were taken in Scotland last year – more than five times the number of train journeys.
The latest report follows government statistics earlier this month that showed a significant increase in the proportion of Scots not leaving their homes on a regular basis, with those over 60 increasingly less mobile than younger people.
SPT said its research demonstrated the need for radical action to improve the attractiveness of bus travel and showed that operators were trapped in a "downward spiral" of cutting bus routes and frequencies in order to reduce costs.
After cutting routes at weekends and in the evenings, companies had now started "thinning out" their networks by reducing the frequency of regular urban services, SPT said.
It has called on the Scottish Government for a major overhaul of urban traffic management in Scotland's cities to ensure bus lanes are given greater priority, and for tougher regulation of the industry to stop firms axing routes or cutting service frequency at short notice.
SPT chairman George Redmond said: "The status quo is no longer an option - SPT has laid out clear plans to improve the bus market and we are working with our partners to make that happen.
"If we are to realise our ambition of getting people on to public transport and boosting the economy, we need to tackle this serious decline now."
The Confederation of Passenger Transport (CPT), which represents bus and coach firms, welcomed SPT's commitment to improving bus priority. A spokesman for CPT said: "Congestion in Strathclyde is markedly worse than the Scottish average and so efforts to free buses from the traffic – and thereby increasing reliability and reducing running times – are to be welcomed."
But he said it was "no surprise" that firms had been forced to make cuts, pointing out that support for subsidised routes had fallen by 10%.
"As well as the problem of congestion, there is an array of cost pressures currently facing the bus industry. The bus service operators' grant (BSOG) has been cut, the budget for concessionary travel is capped at an unsustainable level and fuel costs are rising," he said.
A spokeswoman for the Scottish Government said it was committed to improving bus services and would listen to plans made through its Bus Stakeholder Group, set up in April to tackle the decline in services.
She said: "This Government is absolutely committed to improving bus services for passengers and communities - That is why we are investing £250m every year to support a range of schemes including the BSOG, concessionary travel, Scottish Green Bus Fund and creating a Bus Investment Fund."